Space Travel and Nutrition

Photo by: carroteater

Nutrition
has played a critical role throughout the history of exploration, and
space exploration is no exception. While a one- to two-week flight aboard
the Space Shuttle might be analogous to a camping trip, adequate nutrition
is absolutely critical when spending several months aboard the
International Space Station or several years on a mission to another
planet. To ensure adequate nutrition, space-nutrition specialists must
know how much of various individual
nutrients
astronauts need, and these nutrients must be available in the spaceflight
food system. To complicate matters, spaceflight
nutritional requirements
are influenced by many of the
physiological
changes that occur during spaceflight.

Space Physiology

Spacecraft, the space
environment
, and weightlessness itself all impact human physiology. Clean air,
drinkable water, and effective waste collection systems are required for
maintaining a habitable environment. Without the Earth's atmosphere
to protect them, astronauts are exposed to a much higher level of
radiation than individuals on the Earth. Weightlessness impacts almost
every system in the body, including those of the bones, muscles, heart and
blood vessels, and nerves.

Bone.

Bone loss, especially in the legs, is significant during spaceflight. This
is most important on flights longer than thirty days, because the amount
of bone lost increases as the length of time in space increases.
Weightlessness also increases excretion of
calcium
in the urine and the risk of forming
kidney stones
. Both of these conditions are related to bone loss.

Many nutrients are important for healthy bone, particularly calcium and
vitamin D
. When a food containing calcium is eaten, the calcium is absorbed by the
intestines
and goes into the bloodstream.
Absorption
of calcium from the intestines decreases during spaceflight. Even when
astronauts take extra calcium as a supplement, they still lose bone.

On Earth, the body can produce vitamin D after the skin is exposed to the
sun's ultraviolet light. In space, astronauts could receive too
much ultraviolet light, so spacecraft are shielded to prevent this
exposure. Because of this, all of the astronauts' vitamin D has to
be provided by their
diet
. However, it is very common for vitamin D levels to decrease during
spaceflight.

Sodium intake is also a concern during spaceflight, because space diets
tend to have relatively high amounts of sodium. Increased dietary sodium
is associated with increased amounts of calcium in the urine and may
relate to the increased risk of kidney stones. The potential effect of
these and other nutrients on the maintenance of bone health during
spaceflight highlights the importance of optimal dietary intake.

Bone is a living tissue, and is constantly being remodeled. This
remodeling is achieved through breakdown of existing bone tissue (a
process called resorption) and formation of new bone tissue. Chemicals in
the blood and urine can be measured to determine the relative amounts of
bone resorption and formation. During spaceflight, bone resorption
increases significantly, and formation either remains unchanged or
decreases slightly. The net effect of this imbalance is a loss of bone
mass.

It is not clear whether bone mass lost in space is fully replaced after
returning to Earth. It is also unclear whether the quality (or strength)
of the replaced bone is the same as the bone that was there before a
spaceflight. Preliminary data seem to show that some crew members do
indeed regain their preflight bone mass, but this process takes about two
or three times as long as their flight. The ability to understand and
counteract weightlessness-induced bone loss remains a critical issue for
astronaut health and safety.

The changes in bone during spaceflight are very similar to those seen in
certain situations on the ground. There are similarities to
osteoporosis
, and even
paralysis
. While osteoporosis has many causes, the end result seems to be similar
to spaceflight bone loss. Paralyzed individuals have
biochemical
changes very similar to those of astronauts. This is because in both
cases the bones are not being used for support. In fact, one of the ways
spaceflight bone loss is studied is to have people lie in bed for several
weeks. Using this approach, scientists attempt to understand the
mechanisms of bone loss and to test ways to counteract it. If they can
find ways to successfully counteract spaceflight bone loss, doctors may be
able to use similar methods to treat people with osteoporosis or
paralysis.

Muscle.

Loss of body weight (mass) is a consistent finding throughout the history
of spaceflight. Typically, these losses are small (1 percent to 5 percent
of body mass), but they can reach 10 percent to 15 percent of preflight
body mass. Although a 1 percent body-weight loss can be explained by loss
of body water, most of the observed loss of body weight is accounted for
by loss of muscle and adipose (
fat
) tissue. Weightlessness leads to loss of muscle mass and muscle volume,
weakening muscle performance, especially in the legs. The loss is believed
to be related to a
metabolic
stress associated with spaceflight. These findings are similar to those
found in patients with serious diseases or trauma, such as burn patients.

Exercise routines have not succeeded in maintaining muscle mass or
strength of astronauts during spaceflight. Most of the exercises performed
have been
aerobic
(e.g., treadmill, stationary bicycle). Use of resistance
exercise, in which a weight (or another person) provides resistance to
exercise against, has been proposed to aid in the maintenance of both
muscle and bone during flight. Ground-based studies (not done in space) of
resistance exercise show that it may be helpful, not only for muscle but
also for bone. Studies being conducted on the International Space Station
are testing the effectiveness of this type of exercise for astronauts.

Blood.

A decrease in the mass of red blood cells (i.e., the total amount of blood
in the body) is also a consistent finding after short- and long-term
spaceflight. The actual composition of the blood changes little, because
the amount of fluid (blood
plasma
) decreases as well. The net result is that the total volume of blood in
the circulatory system decreases. While this loss is significant (about 10
percent to 15 percent below preflight levels), it seems to be simply an
adaptation to spaceflight, with no reported effect on body function during
flight.

The initial loss of red blood cells seems to happen because newly
synthesized cells (which are not needed in a smaller blood volume) are
destroyed until a new steady state is reached. One consequence of the
increased destruction of red blood cells is that the
iron
released when they are destroyed is processed for storage in the body.
Too much iron may be harmful, and is thus a concern for long space
missions.

Space Food Systems

Historically, space food systems have evolved as U.S. space programs have
developed. The early Mercury program (1961–1963) included food
packaged in bite-sized cubes, freeze-dried powders, and semiliquid foods
(such as ham salad) stuffed into aluminum tubes.

The Gemini program (1965–1966) continued using bite-sized cubes,
which were coated with plain gelatin to reduce crumbs that might clog the
air-handling system. Freeze-dried foods were put into a special plastic
container to make rehydrating easier.

The Apollo program (1968–1972) was the first to have hot water.
This made rehydrating foods easier, and also improved taste and quality.
Apollo astronauts were the first crew members to use the
spoonbowl,
a utensil that eliminated having to consume food into the mouth directly
from the package.

The quality, taste, and variety of foods improved even more during the
Skylab program (1973–1974), the only program to have refrigerators
and freezers for storage of fresh foods. The menu contained seventy-two
different food items.

The Shuttle program, which began in 1981, includes food prepared on Earth
from grocery store shelves. With the help of a dietitian, crew members
plan individual three-meal-per-day menus that contain a balanced supply of
the nutrients needed for living and working in space. Crew members are
allowed to add a few of their own personal favorite foods (which may
require special packaging to withstand the rigors of spaceflight).
Freezedried foods are rehydrated using water that is generated by the
Shuttle's fuel cells. Foods are eaten right from the package (on
individual food trays), or they may be heated in a convection oven in the
Shuttle galley.

Astronauts on the International Space Station prepare to share a
meal. The quality of their menu contrasts sharply with those of the
early space explorers, whose meals were either
semi-liquids—squeezed from a tube—or bite-sized cubes.

[NASA. Reproduced by permission.]

During the Shuttle-Mir program (1995–1998), a joint menu was used
that contained half Russian and half U.S. Shuttle foods. These had to meet
the nutritional needs established by technical committees representing
both space programs. The Russian four-meal-per-day menu was used, with
each space program providing two of the meals. Three larger meals were
designed to be eaten as scheduled meals; the fourth meal was composed of
foods that could be eaten at any time throughout the day.

A Space Shuttle meal tray includes scissors to cut open food packages
and Velcro to hold them in place. The tray itself is secured to the
wall or to an astronaut's lap to keep it from drifting away.

[NASA. Reproduced by permission.]

The current food system for the International Space Station, which started
in 2000, is similar to the system used in the Shuttle-Mir program. The
four-meal-per-day menu plan is used, with equal provision of foods by the
U.S. and Russian space programs. The menu is composed mainly of packaged
foods that are freeze-dried and thermostabilized (canned), with very few
fresh foods. The crew members plan their own menus with the assistance of
a dietitian, and an effort is made to include all of the nutrients needed
for working in the space environment. After the habitation module galley
is equipped with refrigerators, freezers, and a microwaveconvection oven,
a more extensive menu, including a variety of fresh foods, will be
available.

Dietary Intake during Spaceflight

Dietary intake has been monitored on select Apollo, Skylab, Shuttle, and
Shuttle-Mir flights as a part of scientific studies. Preflight and
postflight intakes are determined using conventional methods for
dietary assessment
. Crew members are provided a diet-record logbook and digital scale, or
the foods are weighed by the research dietitian and provided during each
of the five- to eighteen-day data collection sessions. A variety of
nutrient-analysis software programs are used. Crew members record their
intake during space-flight by writing it in a log or, more frequently,
they use a barcode reader that scans the food package label and then
record the amount consumed. The amounts of certain nutrients in each meal
are calculated from the record of how much of each type of food was eaten,
plus knowledge of the amount of each nutrient in each type of food.
Nutrient calculations using chemical analysis data for each spaceflight
food item are performed after the flight. On the International Space
Station, crew members complete a food-frequency questionnaire each week,
and the data is down-linked for analysis. Dietary intake can thus be
assessed in real time. Changes in diet may then be suggested to the crew
members to prevent nutrient deficiencies.

A primary concern is that astronauts consume enough
energy
(
calories
) for optimal work performance and good health. Of the flight crews that
have been monitored, only the Skylab crew members consumed enough
energy—99 percent of their predicted intake. Most of the crew
members in other flight programs consumed about 70 percent of what was
planned. On the Skylab flights, much time and attention was given to
eating and food preparation, and the crew members' extensive
exercise program may have stimulated their appetite. On all other flights,
the crew members have had a very busy schedule, with little time and
attention devoted to eating.

Crew members' dietary intakes on Skylab, Shuttle, and Shuttle-Mir
flights have tended to be higher in
carbohydrate
and lower in fat than their pre-flight intakes. This change may have been
related to an abundance of foods high in carbohydrates, especially
sugar-sweetened beverages, or perhaps these items are more easily prepared
during a busy work schedule. Ample fat sources are available in the
Shuttle food inventory—more than half of the main dish items
contain greater than 30 percent of their calories as fat.

Intake of fluid should be about 2,000 milliliters (2 liters) per day,
which is sufficient to prevent
dehydration
and kidney stone formation. Fluid intakes have varied from 1,000 to 4,000
milliliters per day, indicating that some crew members are getting less
than the recommended amount.

Inflight sodium intakes of all crew members have exceeded the
recommendation of less than 3,500 milligrams per day. Sodium intake is
high because many of the "off-the-shelf" food items used
have a high sodium content.

Calcium intakes have been below the recommended range of 1,000 to 1,200
milligrams per day. This level is estimated to minimize the bone mineral
loss that occurs during spaceflight.

Iron intakes have been 50 to 60 percent greater than the recommendation of
ten milligrams per day. As with sodium, iron intakes are high because the
food items have already been iron-fortified. Too much iron in the body may
cause tissue damage.

Nutrition is critical for health, both on Earth and during spaceflight.
Specific nutrition concerns for spaceflight include adequate consumption
of calories for energy, adequate fluid intake to prevent dehydration and
renal stones, adequate calcium to minimize bone loss.

There seems to be an excess of both sodium and iron in the inflight diet,
compared to predicted requirements. A food delivery system needs to be
designed to include foods that will provide nutrients at the recommended
levels, while providing variety and palatability to make eating more
pleasant.

The International Space Station represents the beginning of an era of
humans living and working in space, with the potential for a permanent
human presence in space. Nutrition will play a vital role in ensuring the
health and safety of spacefaring individuals, whether they are in low
Earth orbit or on journeys to the moon, Mars, or beyond. A more complete
understanding of the effects of spaceflight will not only help humans to
explore the universe, but will provide information needed to maintain
human health and treat diseases here on Earth.